Optical communications networks are used to transport large amounts of voice and data through fiber optic cables. Wave-division multiplexing (WDM) can be used to transmit multiple channels of optical information through the same fiber optic cable. Multiple channels are carried in the same fiber by assigning different optical wavelengths to each channel.
The optical signals degrade over distance such that re-generation or re-amplification may be required. Moreover, the channels within a fiber tend to degrade at different rates. The total mean output power is frequently used as an indicator of signal degradation. Optically amplifying the signals to maintain the total mean output power at a constant is not particularly effective due to the differences in degradation between channels. Moreover, increasing the amplification gain to maintain a constant mean output power without regard to individual channels can create significant distortions on an individual channel basis. Increasing gain to compensate for the loss of a few channels may result in too much amplification for the remaining channels individually even though the total mean power output remains constant. Accordingly, determination and regulation of optical power on a channel-by-channel basis is desirable.
In order to measure optical power on a channel-by-channel basis, an identifying pilot tone is superimposed upon each channel. Within a given fiber the pilot tones are unique in order to distinguish all channels for the purpose of power calculations. Given that the pilot tone and the channel signal originate from the same source, the optical channel power can be inferred from the pilot tone power and its fixed modulation depth at the source. The optical channel power can then be used to adequately control the optical amplifiers in the network.
One technique for setting the pilot tone power and a modulation depth at the source uses an open loop control. Open loop controls, however, cannot automatically accommodate optical sources such as laser diodes that have operating characteristics that may vary significantly with age. Differences between the actual and presumed values of the pilot tone power and modulation depth at the source can significantly affect the accuracy of the estimated or inferred optical channel power thus inhibiting efficient operation of the communications network.